The present invention relates to a silicone composition and more particularly to a hydrosilylation addition-curable silicone resin composition comprising a silicone resin, an organohydrogensilane, and an inorganic filler. The present invention further relates to a cured silicone product formed from the composition.
Silicones are useful in a variety of applications by virtue of their unique combination of properties, including high thermal stability, good moisture resistance, excellent flexibility, high ionic purity, low alpha particle emissions, and good adhesion to various substrates. For example, silicones are widely used in the automotive, electronic, construction, appliance, and aerospace industries.
Silicone compositions comprising an organopolysiloxane resin organohydrogensiloxane, and hydrosilylation catalyst are known in the art. The organopolysiloxane resin is typically a copolymer containing T and/or Q siloxane units in combination with M and/or D units. However, such compositions cure to form silicone products having relatively low fracture toughness and high coefficients of thermal expansion, rendering them unsatisfactory for certain applications, such as attachment of heat sinks to semiconductor chips and semiconductor chips to substrates. During thermal cycling, stresses develop in the silicone product due to differences in the coefficients of thermal expansion between the substrate materials and the silicone. Thermally induced stresses can weaken or fracture the bond joint, limiting the useful service life of a device.
Various approaches to improving the fracture toughness of cured silicone resins are reported in the literature. For example, U.S. Pat. No. 5,747,608 to Katsoulis et al. discloses a rubber-modified rigid silicone resin comprising a copolymerized reaction product of an organosilicone resin and a silicone rubber. The ""608 patent discloses, inter alia, addition-curable silicone resin compositions.
Also, European Patent Application EP 1 065 248 A1 to Katsoulis et al. discloses a hydrosilylation reaction curable composition comprising a silsesquioxane copolymer, a silyl-terminated hydrocarbon, and a hydrosilylation reaction catalyst. The ""248 application also teaches that the cured silsesquioxane resin has improved strength and toughness without significant loss of modulus.
Inorganic fillers such as silica and alumina are typically added to silicone compositions to reduce the coefficient of thermal expansion of cured products. Such fillers have very low coefficients of thermal expansion compared to the cured silicone products. However, the addition of moderate to high amounts of inorganic fillers to conventional silicone resin compositions can significantly reduce the fracture toughness of the cured resins.
Consequently, there is a need for a hydrosilylation addition-curable silicone composition that cures to form a silicone product having a low coefficient of thermal expansion and high fracture toughness.
The present invention is directed to a silicone composition comprising:
(A) 100 parts by weight of an organopolysiloxane resin containing an average of greater than two alkenyl groups per molecule and having less than 1.5 mol % of silicon-bonded hydroxy groups, wherein the resin is selected from:
(a) a copolymer consisting essentially of R23SiO1/2 units and R1SiO3/2 units, wherein the mole ratio of R23SiO1/2 units to R1SiO3/2 is from 0.05 to 3.0,
(b) a copolymer consisting essentially of R23SiO1/2 units, R1SiO3/2 units, and SiO4/2 units, wherein the mole ratio of R23SiO1/2 units to R1SiO3/2 is from 0.05 to 3.0 and the mole ratio of R23SiO1/2 and R1SiO3/2 units combined to SiO4/2 units is from 4 to 99,
(c) a copolymer consisting essentially of R23SiO1/2 units, R1SiO3/2 units, and R22SiO2/2 units, wherein the mole ratio of R23SiO1/2 units to R1SiO3/2 units is from 0.05 to 3.0 and the mole ratio of R23SiO1/2 units and R21SiO3/2 units combined to R22SiO2/2 units is from 0.5 to 99; and
(d) a copolymer consisting essentially of R22SiO2/2 units and R1SiO3/2 units, wherein the mole ratio of R22SiO2/2 units to R1SiO3/2 units is from 0.2 to 4.0; wherein R1 is a hydrocarbyl group free of aliphatic unsaturation and R2 is selected from R1 and alkenyl;
(B) an organohydrogensilane in an amount sufficient to cure the composition, the organohydrogensilane having the formula: 
wherein R3 is a hydrocarbyl group free of aliphatic unsaturation and R4 is a hydrocarbylene group having a formula selected from: 
wherein x is from 1 to 6;
(C) an effective amount of an inorganic filler; and
(D) a catalytic amount of a hydrosilylation catalyst.
The present invention is further directed to a cured silicone product comprising a reaction product of the above-described composition.
The present invention is still further directed to a multi-part silicone composition comprising components (A) through (D) in two or more parts, provided components (A), (B), and (D) are not present in the same part.
The silicone composition of the present invention has numerous advantages, including low VOC (volatile organic compound) content and adjustable cure. Moreover, the silicone composition cures to form a silicone product having low CTE and superior fracture toughness.
The silicone composition of the instant invention has numerous uses, particularly in the electronics field. For example, the silicone composition can be used to attach a die to a printed circuit board, encapsulate an electronic device, fill the gap between a heat sink and an electronic device, attach a heat sink to an electronic device, or encapsulate the wire windings in a power transformer or converter. In particular, the silicone composition is useful for bonding electronic components to flexible or rigid substrates.